1. Field of the Invention
The present invention is directed to a gas to liquid mass transfer device. More particularly, the present invention is directed to a mass or heat transfer device wherein large quantities of a gas composition of an industrial or chemical process or the like are efficiently treated with a liquid composition usually for the purpose of chemically or physically absorbing a component of the gas composition from the liquid composition.
2. Brief Description of the Prior Art
Devices for absorbing one or several components of a gas composition have been known in the art for a long time. Such devices, commonly termed scrubbers, operate on the prin ciple that a stream of the gas composition is mixed with a finely dispensed spray, mist, or stream of the "scrubbing" liquid composition which absorbs the desired component of the gas either by a chemical reaction or by simple physical absorption.
As it is readily appreciated by those skilled in the art, the "scrubbing" or mass transfer process occurs on the interfacing gas liquid surfaces. Therefore an efficient mass transfer process requires thorough intermixing of the gas and liquid phases. Similarly, efficiently intermixed liquid and gas phases may also result in efficient heat or momentum (kinetic energy) exchange between the two phases.
Among the several devices utilized in the prior art for the above-noted purpose, scrubbers utilizing the venturi principle, or the countercurrent principle are of major interest as background to the present invention.
Briefly, scrubbers utilizing the countercurrent principle usually include a tower or like structure wherein the gas composition flows in an upwardly direction, and a suitable scrubbing liquid falls in the form of a fine spray or mist in a downwardly direction. In other scrubbers of the prior art which operate on the countercurrent principle, the scrubbing liquid "percolates" in a downward flow through loose packing material of the tower.
Scrubbers which utilize the venturi principle usually include a substantially tubular body having an appropriately varying cross-section along the longitudinal axis of the body. The tubular body is in fluid communication with a source of the gas composition. A stream of the suitable scrubbing liquid is sprayed into the tubular body under pressure, usually from a spray head centered on the longitudinal axis of the the tubular body. The tubular body is of a gradually decreasing cross-section downstream of the spray head, to provide a throat. Downstream of the throat the tubular body forms a diffuser of gradually increasing cross-section. Motive power of the device is provided by kinetic energy of the liquid sprayed into the device under pressure.
As is well known, the above-summarized, and other prior art scrubbing or gas-to-liquid-mass-transfer devices function acceptably in certain applications. Nevertheless, difficulties are known to arise often, when a reasonably functioning scrubbing device is used as a model for scaling-up for a larger sized scrubbing operation.
As examples, difficulties which arise in the prior art in connection with certain sulfur recovery processes are noted. Briefly, in these processes the hydrogen sulfide (H.sub.2 S) content of a gas composition (usually the tail gas of a hydrocarbon refining process) is extracted by chemical reaction with an appropriate chemical reagent solution. After the extraction or scrubbing is complete, the gas composition is usually discarded by release into the environment.
It is readily apparent from the foregoing that, for environmental and economic reasons, the above-noted hydrogen sulfide extraction processes must be highly efficient. Furthermore, a multitude of other commercially important, large scale chemical manufacturing processes exist wherein highly efficient removal of a gaseous component of a carrier gas composition with a suitable scrubbing liquid, is of similar importance.
While attempting to increase the scale of the gas-to-liquid-mass transfer or scrubbing process, particularly in the above-noted sulfur recovery processes, the prior art confronted the very serious problem of significantly reduced efficiency with increasing size of the above-noted venturi type scrubbing apparatus. More particularly, efficiency (.eta.) of the gas-to-liquid scrubber (as applied to the sulfur recovery process) is defined in Equations 1 and 2. EQU .eta.=1-1/E Equation 1
wherein E is the extraction ratio. Thus, E is the ratio of hydrogen sulfide concentration in the gas composition entering the scrubber to the hydrogen sulfide gas concentration in the gas composition leaving the scrubber. ##EQU1##
It is generally accepted in the art that, for environmental and economic reasons, in a sulfur recovery process the extraction ratio should be at least approximately one thousand (1000) (.eta.=0.999).
Prior art venturi scrubbers are, by and large, capable of operating with an efficiency of 0.999 (E=1000) as long as the scrubber is relatively small. However, efficiency deteriorates rapidly when the size and operational capacity of the prior art venturi scrubber is increased by providing a larger throat diameter, or when several liquid ejectors or nozzles spray liquid into a throat of increased diameter.
Similar problems of reduced gas-to-liquid mass transfer efficiency are experienced in other processes, where in accordance with the prior art, large, industrial scale scrubbers or mass transfer devices are constructed.
Accordingly, there is a need in the prior art for an efficient gas-to-liquid mass transfer device which is adaptable for practically any scale of operation without adverse effect on its efficiency. The present invention provides such a mass transfer device.